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AMD Alchemy™ Processors Building a Root File System for Linux® Incorporating Memory Technology Devices

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AMD Alchemy™ Processors Building a Root File System for Linux® Incorporating Memory Technology Devices 1.0 Scope This document outlines a step-by-step process for building and deploying a Flash-based root file system for Linux® on an AMD Alchemy™ processor-based development board, using an approach that incorporates Memory Technology Devices (MTDs) with the JFFS2 file system. Note: This document describes creating a root file system on NOR Flash memory devices, and does not apply to NAND Flash devices. 1.1 Journaling Flash File System JFFS2 is the second generation of the Journaling Flash File System (JFFS). This file system provides a crash-safe and powerdown-safe Linux file system for use with Flash memory devices. The home page for the JFFS project is located at http://developer.axis.com/software/jffs. 1.2 Memory Technology Device The MTD subsystem provides a generic Linux driver for a wide range of memory devices, including Flash memory devices. This driver creates an abstracted device used by JFFS2 to interface to the actual Flash memory hardware. The home page for the MTD project is located at http://www.linux-mtd.infradead.org. 2.0 Building the Root File System Before being deployed to an AMD Alchemy platform, the file system must first be built on an x86 Linux host PC. The pri- mary concern when building a Flash-based root file system is often the size of the image. The file system must be designed so that it fits within the available space of the Flash memory, with enough extra space to accommodate any runtime-created files, such as temporary or log files. Before moving on to the deployment phase, the root file system should be tested using either an NFS-based or hard disk- based configuration to ensure its functionality. 2.1 OpenEmbedded Root File System A root file system generated by the OpenEmbedded (OE) toolkit is often suitable for deployment onto a Flash device with- out any major modifications. The AMD Alchemy™ Au1200™ OE Development Kit is configured to build a BusyBox-based system by default, and it is not recommended that this be changed unless absolutely necessary. To ensure no space is wasted, include only those packages necessary for operation of the system in the platform’s OE meta-package before building the image. To follow the steps for deployment, use the generated root file system located in the travis-dev-kit/tmp/rootfs directory if using the directory structure outlined in the AMD Alchemy™ DBAu1200™ Develo- pent Board OpenEmbedded Quick Start Guide (Publication ID 40351). 2.2 MontaVista Root File System The MontaVista Pro toolkit includes a very large pre-built root file system. This file system must be reduced using an itera- tive process until its footprint fits into the available Flash memory. It is highly recommended that the file system be deployed using NFS during the reduction process, as any changes can be tested immediately. 2.2.1 BusyBox The first step in reducing the footprint of the MontaVista root file system is to switch from the default Bash shell and coreutils to a BusyBox-based system. BusyBox is a single multi-call binary that replaces and replicates the basic and most- used functionality of the shell and many of the necessary utilities for running a Linux system. The BusyBox binary is approx- imately 1 MB in size, and can be built to rely on no external libraries except for the C library, libc. 40755A - May 2006 - Confidential 1 Application Note 40755A - May 2006 - Confidential The BusyBox source code and documentation can be obtained at http://www.busybox.net. While BusyBox advocates the use of uClibc, a smaller version of the C library, it is entirely possible, and often desirable, to build BusyBox to use the pre- compiled C library contained within the original MontaVista root file system. 2.2.2 Removing Unnecessary Packages The most time consuming portion of reducing the MontaVista footprint is removing the packages that are not necessary for the target system. This can be done in a top-down manner, by removing a subset of the unnecessary packages and then testing the file system, or in a bottom-up manner by starting with only BusyBox and adding additional necessary packages one-at-a-time and resolving dependency errors that arise. The method chosen depends largely on developer preference. In either case, it is helpful to use the ldd tool (ensure a MIPS-based tool is used (e.g., mips_nfp_le-ldd)) to determine the library dependencies of a dynamically-linked binary executable. 2.2.3 Removing Debug Information The MontaVista root file system binaries and libraries are built with debugging symbols included. While this is beneficial when debugging software, it can also greatly increase the size of the executable. Once the file system has been reduced to its necessary functionality, the debugging information can be removed from the included binaries, further reducing the foot- print of the file system. This is done using the strip tool, as shown in the following example, which removes the debugging symbols from the C library: # mips_nfp_le-strip -d libc-2.3.2.so 3.0 Deploying a JFFS2 File System Once the file system has been built on the x86 host PC and tested on the target platform, the file system must then be con- verted into a JFFS2 image and written to Flash. 3.1 Add MTD Support to the Linux Kernel The Linux kernel must be configured to support MTD and JFFS2. Since the root file system will be mounted using JFFS2, all of the kernel options must be enabled as built-ins, not as modules. For a Linux 2.4 kernel on a AMD Alchemy™ DBAu1550™ development board, enable the following options: Memory Technology Devices (MTD) ---> <*> Memory Technology Device (MTD) Support <*> MTD Partitioning Support RAM/ROM/Flash chip drivers ---> <*> Detect flash chips by Common Flash Interface (CFI) probe <*> Support for AMD/Fujitsu Flash Chips Mapping drivers for chip access ---> <*> Db1550 MTD Support File systems ---> <*> Journalling Flash File System v2 (JFFS2) support For a Linux 2.6 kernel on a AMD Alchemy™ DBAu1200™ development board, enable the following options: Device Drivers ---> Memory Technology Devices (MTD) ---> <*> Memory Technology Device (MTD) support [*] MTD partitioning support RAM/ROM/Flash chip drivers ---> <*> Detect flash chips by Common Flash Interface (CFI) probe <*> Support for AMD/Fujitsu flash chips Mapping drivers for chip access ---> <*> AMD Alchemy Pb1xxx/Db1xxx/RDK MTD support File systems ---> Miscellaneous filesystems ---> <*> Journalling Flash File System v2 (JFFS2) support 2 AMD Alchemy™ Processors Building a Root File System for Linux® Incorporating Memroy Technology Devices Application Note 40755A - May 2006 - Confidential The MTD mappings for the AMD Alchemy development boards can be found within the Linux source tree in driv- ers/mtd/maps/. For the examples above, the actual filenames are db1550-flash.c and alchemy-flash.c, respectively. These MTD mappings divide the Flash memory into separate regions of differing size as shown in Figure 3-1. These regions become separate MTD partitions when Linux is run. In the case of the AMD Alchemy development boards, the regions are arranged as follows. 3 MB mtdblock2 Kernel image (raw binary) 1 MB mtdblock1 YAMON (raw binary) Size -4 MB mtdblock0 Root file system (JFFS2) Figure 3-1. MTD Partition Mapping Therefore, on a DBAu1200 development board, which has 64 MB of Flash, the lower 60 MB of Flash is available for use as the root file system, while the upper 4 MB is reserved for the YAMON boot loader and a 3 MB binary kernel image. Note, however, that YAMON uses the upper 256 kB of Flash to store its environment variables, so this space is not made acces- sible by MTD. 3.2 Obtain MTD Tools In order to build a JFFS2 image, the mkfs.jffs2 tool must be built from the MTD toolkit. Obtain the MTD source directly from the project’s CVS server: # cvs -d :pserver:[email protected]:/home/cvs login password: anoncvs # cvs -d :pserver:[email protected]:/home/cvs co mtd Alternately, obtain daily snapshots from ftp://ftp.uk.linux.org/pub/people/dwmw2/mtd/cvs/. Once the MTD source has been obtained, build the mkfs.jffs2 utility (the actual drivers have already been incorporated into the AMD Alchemy Linux kernel source tree, and need not be built): # cd mtd/util # make mkfs.jffs2 Once the binary is built, it may be copied to any location within the PATH. # cp mkfs.jffs2 /usr/sbin 3.2.1 Prepare Root File System Before booting the root file system, ensure the /etc/fstab file is configured to use an MTD-based root file system. Create or replace the existing entry for the root mount point (/) in the /etc/fstab file with an entry similar to the following: /dev/mtdblock0 / jffs2 defaults,sync,noatime 1 1 Unless using devfs or udev, MTD device nodes must also be added to the /dev/ directory of the root file system. This can be done using the MAKEDEV script included in the MontaVista root file system: # cd rootfs/dev # ./MAKEDEV mtd # ./MAKEDEV mtdblock AMD Alchemy™ Processors Building a Root File System for Linux® Incorporating Memroy Technology Devices 3 Application Note 40755A - May 2006 - Confidential 3.3 Create JFFS2 Image Once the mkfs.jffs2 tool has been built, it can be used to build a JFFS2 binary image out of the root file system created in Section 2.0 on page 1. The following example creates an image for a DBAu1200 development board from a file system located in the directory “rootfs”. # mkfs.jffs2 –p0x3C00000 –drootfs –orootfs.jffs2 –l –q –v The image created, rootfs.jffs2, will be 60 MB (0x3C00000 bytes), which is the size of the available MTD partition as dis- cussed in Section 3.1 beginning on page 2.
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